RESEARCH OBJECTIVES: The proposed research is a continuation of an analysis of primate motor control systems in which the primary emphasis is on motor cortex modulation of the spinal motor systems. We plan to continue our study on the nature of feedback signals from the muscle spindle to the cerebral cortex. The research will focus on muscle afferent projections to Areas 3a, 4, 5, and 6. Of particular interest is the measurement of latency, threshold, and position and velocity sensitivities of cortical neurons. This will be accomplished by recording cortical unit discharge during stimulation of muscle nerves and during different rates and amplitudes of muscle stretch. The influence of muscle afferent input on cortical neurons whose axons descend back to the spinal cord will be studied. Since the projections from area 3a remain obscure and since this is the principal cortical receiving area for information from primary spindle afferents, we will attempt to define the interconnections of 3a with other areas of the central nervous system. Finally, the functional interrelationships between sensory receiving areas and motor areas of the cortex will be studied for the purpose of better defining the possible mechanisms involved in motor control. Primary emphasis, again, will be on signals from muscle spindles which inform the cortex about the length and changes in length of the muscles whose contractions make possible motor behavior.